1. Field of the Invention
The invention is related to a transmission system comprising a transmitter for transmitting via a transmission medium to a receiver, a carrier modulated by a sequence of digital symbols according to an output constellation having a predetermined number of phase states, said transmitter comprising a cascade connection of a phase modulator and a frequency multiplier.
The invention is further related to a transmitter for transmitting digital symbols modulated on a carrier.
2. Description of the Related Art
A system according to the preamble is known from the Japanese Patent Application laid open under No. 53-110355.
Transmission systems for transmitting digital symbols modulated on a carrier according to a constellation having a predetermined number of phase states are used for a large number of applications. Examples of such applications are mobile radio, radio relay systems, and broadcast systems like Microwave Video Distribution Systems (MVDS). The constellation can vary from BPSK (Binary Phase Shift Keying) or QPSK (Quaternary Phase Shift Keying) to 16.64 and even 256 QAM (Quadrature Amplitude Modulation).
A usual way of generating such a modulated carrier is to modulate a carrier having a fairly low intermediate frequency according to the desired constellation. The desired RF signal, having a frequency that can be in the GHz range (e.g. 27 GHz, 40 GHz), is then obtained by mixing the modulated carrier with a local oscillator signal having a suitable frequency. If the desired frequency is substantially higher than the intermediate frequency complex filters are required to suppress the undesired sideband in the output signal of the mixer. If the desired frequency is above 20 GHz, building blocks such as amplifiers and mixers are required that are able to handle these high frequencies. Such building blocks are difficult to manufacture and are consequently rather expensive.
In the transmission system according to the previously mentioned Japanese Patent Application a cascade connection of a phase modulator and a frequency multiplier is used. The phase modulator modulates a carrier according to a constellation with two phase states .pi./2 radians apart. By using a frequency doubler, a signal modulated according to a constellation with two phase states .pi. radians apart is obtained. This signal is a common BPSK signal. By using a frequency multiplier to obtain the desired RF signal, only the frequency multiplexer must be able to operate at the desired frequency. The remaining building blocks like amplifiers and mixer have only to operate at a fraction of the desired frequencies, and can consequently be less expensive.
Simulations of the transmission system according to the principle of the Japanese Patent Application show a substantial degradation of the bit error rate in comparison with a system that does not use a frequency multiplier for obtaining the final constellation.